Like all non-LTS releases, Ubuntu 15.10 (Wily Werewolf) was supported for 9 months and this period ends today. This means it will no longer receive maintenance updates, including critical security upgrades.

If you're still using Ubuntu 15.10, it's recommended you upgrade to Ubuntu 16.04, which is supported until April 2021. For how to upgrade to Ubuntu 16.04, see THIS page.